Piezo-electric actuators are used in imaging systems to adjust the positions of zoom and focusing lens systems. Piezo-electric actuators are also used in other systems to move small objects. Driving devices providing suitable driving voltages are required to operate said piezo-electric actuators. Piezo-electric actuators are implemented by using one or more piezo-elements, which are deflected, expanded or contracted when coupled to an activating voltage.
An aspect in mobile devices is that the voltage of an available power supply is typically lower than the optimum driving voltage of the piezo-elements. Driving voltages higher than the power supply voltage are typically generated using voltage step-up means, which are coupled to the piezo-elements to provide charge transfer to and from said voltage step-up means. Further, in mobile systems, the available power supply is typically a battery with a limited capacity. Therefore energy consumption related to the driving of piezo-elements is a problem especially in mobile systems.
A piezo-element comprises a substantial internal capacitance, which stores energy when the piezo-element is charged by coupling to a driving voltage. U.S. Pat. No. 6,563,251 discloses a driving device for an actuator having a capacitive motor phase. Said patent teaches that energy liberated from a piezo-element during discharging operations may be stored to be used in subsequent charging operations. Said patent discloses the use of a set of voltage sources to charge/discharge a piezo-element in an energy-efficient way. At least one of the voltage sources may be capacitively buffered to provide a further energy-saving feature. A voltage step-up device is used to increase the driving voltage.